The history of US immigration policy and practice reflects a series of attempts to address complex political demands and organizational tensions. Yet this complexity has rendered comprehensive immigration reform elusive in recent decades. When legislative action appears impossible, what other avenues are available to confront these challenges? During the first term of the Obama administration, prosecutorial discretion emerged as a key mechanism. This article draws on archival data and interviews with immigration attorneys, advocates, analysts, and policy makers to understand better how prosecutorial discretion is used in immigration policy and practice today, why it came to have such a central role, recent challenges to its use, and what these tensions suggests for sociolegal scholarship on immigration. © 2014 American Bar Foundation.